A lithium-ion or sodium-ion battery comprises at least a negative electrode, a positive electrode, a separator and an electrolyte. The electrolyte is composed of a lithium or sodium salt dissolved in a solvent which is generally a mixture of organic carbonates, in order to have a good compromise between the viscosity and the dielectric constant.
The most widely used salts include lithium hexafluorophosphate (LiPF6), which has many of the numerous qualities required but exhibits the disadvantage of decomposing in the form of hydrofluoric acid gas. This presents safety problems, in particular in the context of the impending use of lithium-ion batteries in private vehicles.
Other salts have thus been developed, such as LiTFSI (lithium bis(trifluoromethanesulphonyl)imide) and LiFSI (lithium bis(fluorosulphonyl)imide). These salts exhibit little or no spontaneous decomposition and are more stable with regard to hydrolysis than LiPF6. Nevertheless, LiTFSI exhibits the disadvantage of being corrosive with regard to aluminium current collectors, which is not the case with LiFSI. Thus, LiFSI appears to be a promising alternative to LiPF6.
There exist several known synthetic routes for the manufacture of lithium bis(fluorosulphonyl)imide. One of these routes consists of the reaction of perfluorosulphonic acid with urea: see the document WO 2010/113483 in this regard. The products from this reaction are subsequently dissolved in water and bis(fluorosulphonyl)imide is precipitated in the form of a salt with tetrabutylammonium. However, this synthetic route is not viable on a large scale as the overall yield is very low.
Another route consists in reacting difluorosulphoxide with ammonia: see the document WO 2010/113835 in this regard. However, this method also forms numerous by-products, which requires expensive purification stages.
Furthermore, the document WO 2009/123328 describes generally the manufacture of sulphonylimide compounds. The document describes in particular the reaction between amidosulphuric acid and thionyl chloride, then with chlorosulphonic acid, to form bis(chlorosulphonyl)imide, which is subsequently subjected to a fluorination stage. However, bis(chlorosulphonyl)imide is an unstable compound which does not tolerate purification. For this reason, the impurities present are retained up to the end of the fluorination stage and render the separation more difficult.
Consequently, there exists a real need to develop a process which makes it possible to obtain LiFSI or NaFSI in a simpler way and/or with a better yield.